1. Priority Information
This application claims priority from European Patent Application No.03075431.1, filed Feb. 14, 2003, herein incorporated by reference in its entirety.
2. Field of the Invention
The present invention relates to lithographic apparatus lithographic apparatus comprising a device for wafer alignment with reduced tilt sensitivity.
3. Description of the Related Art
Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device may generate a desired circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist).
Generally, such apparatus include an illumination system for supplying a projection beam of radiation, a support structure for supporting the patterning device, a substrate holder for holding a substrate, and a projection system for projecting the patterned beam onto a target portion of the substrate. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
The term “patterning device” as employed herein should be broadly interpreted as referring to a mechanism that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such a patterning device include:                mask: the concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired;        programmable mirror array: an example of such a device is a matrix-addressable surface having a visco-elastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and        programmable LCD array: an example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.        
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning device as set forth above.
In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic apparatus—commonly referred to as a wafer stepper—each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Because, typically, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic apparatus, the pattern is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer.
If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, both incorporated herein by reference.
For a lithographic process the alignment between the wafer to be processed and the mask pattern on the mask should be as precise as possible for a correct definition of features on the substrate, particularly when the features are subject to pre-specified sizes having specified tolerances. To this end, the lithographic apparatus comprises a wafer alignment module, which provides for alignment of the substrate with the mask and mask pattern within a given (specified) tolerance. The wafer alignment system typically performs the alignment based on optical means.
The position of a wafer or a portion of a wafer is determined by measuring an optical response from an optical marker which is illuminated by an optical source. For example, a grating is illuminated by a laser beam, the laser beam diffracts from the grating, and one or more of the diffracted orders are measured by respective sensors (for example a detector array), which are typically located on a reference plane. Using the output of the sensors the position of the wafer can be derived (relative to the reference plane).
In the prior art wafer alignment systems based on gratings using a Keplerian telescope are known. U.S. Pat. No. 4,251,160 discloses a wafer alignment system, which comprises a Keplerian telescope for imaging diffracted beams generated by a grating on one or more detectors to obtain information on the alignment of a wafer relative to a reference.
A signal from a marker on a wafer (portion) is projected by the telescope system on the reference plane. To fulfill the imaging condition, the object plane (where the marker is located) should be the conjugate plane of the image. Disadvantageously, a marker on a wafer which is not in the object plane, shows a shift in the reference plane, when the marker is tilted relative to the object plane. The shift is due to the defocus of the marker, a tilted marker in focus (i.e., in the object plane) will not display a shift in the reference plane of the wafer alignment system.
Moreover, in a prior art Keplerian telescope, a diaphragm aperture (i.e., a pinhole) is applied which is positioned in between the telescope lenses. The diaphragm serves to reduce stray light from reaching the sensor and, as a consequence, parallax of a projected image. WO97/35234 discloses a wafer alignment system having a diaphragm which comprises a plurality of pinholes which are located at predetermined positions in the plane of intermediate focus where the focus of each diffraction order is expected in the ideal case of an untilted grating. In the prior art this arrangement is used for spatial filtering of the diffraction orders to obtain information from each individual order.
As noted above, during semiconductor manufacturing processes, a wafer is subjected to a plurality of treatments such as annealing, etching, polishing, etc., which may likely cause a roughness of a marker (a recessed area in the marker and/or warping of the marker). Such marker roughness may cause an uncontrolled (local) tilt of the marker and, consequently, a shift of the marker image on the reference plane for a defocused marker. The combination of tilt and defocus causes a position error of the image which may contribute to an overlay error in the construction of a semiconductor device. For suppression of the undesired shift the reference marker must be placed at the object plane with great accuracy (focus calibration). Such focus calibration is non-trivial as will be appreciated by persons skilled in the art.
Typically, a prior art wafer alignment system would have a defocus in the order of about hundred micrometers. A grating (i.e., a marker) on a wafer would typically display a tilting angle in the order of at least 100 μrad. Such tilting angle is due to a large extent due to the surface flatness quality of the wafer, which displays some roughness caused by the manufacturing process. Thus, the tilting angle may vary randomly, the Figure given above is an estimate of the average.
In a prior art wafer alignment system with such values for defocus and tilting angle, the position error or accuracy of alignment is about 20 nanometers (or 3.5 μm/degree).
A tilt of a marker will for a given diffraction order produced by the marker cause a shift of the diffraction angle (relative to the situation of a tilt-free marker). When using a diaphragm, this will lead to a displacement of the diffracted beam (for each diffraction order) relative to the pre-determined pinholes.
Further, in a wafer alignment system using a grating with multiple diffracted beams (diffraction orders) and/or multiple colors, the images of the diffraction orders and/or colors are usually not projected in the same plane due to optical aberrations (“focus differences”). When multiple diffraction orders and/or colors are measured simultaneously, marker roughness results in order-to-order and/or color-to-color differences, respectively, in measured positions of the images, thereby degrading performance of the alignment procedure. Depending on the respective values of defocus and tilting angle for each individual order and/or color, some orders may not be usable.
For improvement of wafer alignment with reduced tilt sensitivity in lithographic apparatus, a prior art Abbe arm calibration system for a Keplerian telescope is disclosed in U.S. Publication No. 2001/0008273 A1, which is relatively complex and cost-ineffective.
Tilt sensitivity is defined as the proportionality between a tilt of an object and a tilt of an image of that object.
A more detailed explanation of the relation between wafer tilt and the sensitivity of detection of the diffracted beams will be described below in the description of embodiments according to the present invention.